Commercial and industrial storage tanks are widely used for storing a great variety of liquids. Some of these liquids are highly corrosive and/or are flammable. The service life of a storage tank will vary, depending upon environmental conditions, including the liquid being stored. Eventually, however, the tank will become corroded and develop leaks. This can result in a significant danger to the environment and health of nearby residents. For example, storage tanks are commonly used for storing gasoline at service stations. Gasoline, of course, is highly flammable and is capable of posing a significant health and safety hazard if not properly contained. Federal as well as local regulations govern he structure of such storage tanks.
Heightened public awareness of the danger posed by storage tanks (particularly underground gasoline storage tanks) has led to additional governmental regulations. Recent proposed regulations will soon require most storage tanks to have secondary containment means and possibly a fail safe design feature to guard against accidental soil, water, and air contamination. Secondary containment means must be capable of containing leaked liquid from the storage tank. Rigid double walled tanks have been suggested as one alternative. While effective for containment purposes, such tanks, as presently available, are costly to build and difficult to install because of their weight. Such tanks are built by basically forming two rigid tanks utilizing different sized, reusable molds and then placing one tank inside the other.
Single and double walled tanks made from fiberglass reinforced resinous material are built using a number of distinct time consuming steps. In all known methods, a cylindrical-shaped, reusable mold is used to build tank halves which are subsequently assembled. Initially, layers of fiberglass followed by a resinous coating are applied to the mold or chopped fiberglass/resin streams are simultaneously directed onto the mold and subsequently cured. Sufficient applications of the fiberglass and resin are made until a wall thickness is obtained which has the desired strength. Next, support rib molds of cardboard, four to six inches wide, are placed completely around the cylinder at approximately sixteen inch intervals. Fiberglass and resin are then applied over the cardboard molds and onto adjacent areas of the cylinder so as to become an integral part of the inner tank shell. The mold is finally removed. The cylindrical-shaped wall, including the ribs and one end of the tank, are produced in this stage of the method. The above steps are repeated to obtain a second half-tank. The two half-tanks are then joined together by appropriate sealing means. The resultant single walled tank is capable of being installed in the ground and, in fact, is of the type which has been extensively used for the past twenty years.
In more recent years, double walled tanks have been built and used. Essentially, these tanks are built by the same method as the single walled tanks. An inner, rigid tank is formed in the above described manner. Next, a larger diameter reusable mold is used to build a horizontal half-tank. The fiberglass/resin is applied in a known manner to the mold and cured to form the half-tank. A second horizontal half-tank is formed. Next, the completed inner tank is placed into the larger diameter half-tank. The ribs on the inner tank are properly dimensioned to act as spacer ribs between the two tanks. The second larger diameter half-tank is placed over the inner tank, joined and sealed at the seams with its matching halftank. The resultant product is a double walled storage tank system comprised of essentially two rigid tanks, one inside the other.
A second method of making double walled fiberglass, reinforced, resinous tanks is similar to the above method and us just as time consuming and costly. In this method, the mold has a design wherein the ribs are formed as the fiberglass and resin material is applied. After forming the inner tank of which the ribs are an integral part thereof, the mold is removed. The interior portion of the tank next has a fiberglass/resin layer applied over the rib indentations to result in a smooth cylindrical-shaped interior. A second half-tank is formed in the same manner and the two halves joined. A cylindrical-shaped outer tank is then formed in horizontal halves. The formed inner tank and outer tank halves are assembled as in the first method described above to form a double walled storage tank system based on two rigid tanks with support ribs therebetween.
As is readily apparent, building a double walled storage tank system by known methods is very labor extensive and costly. Recent concerns about leaked tanks has heightened the need for an efficient and economical manner of building double walled storage tank system. A jacketed storage tank system, as disclosed in my U.S. Pat. No. 4,523,454 also provides secondary containment means and avoids the problems associated with the rigid double walled systems. Additionally, the aforementioned jacket system features a fail-safe design due to the fact it provides continuous monitoring means whereby the integrity of both the primary and secondary containment means are checked to insure that leakage of either containment means is known when it first occurs.
Lacking in current designs of jacketed tanks with flat ends is the ability of the jacket to withstand the forces created on the fiberglass jacket's ends when the monitoring space is filled with a liquid or pressure is applied. To install domed end caps over the end of flat ended tanks is not practical or cost effective. Tanks ranging from six feet to eleven feet in diameter with domed end caps would require from 500 to 1,000 gallons of detection liquid just to fill the end caps. Further the spherical end caps would add considerable more length to the tank which is a disadvantage with shipping or installation underground
Currently-built double walled fiberglass tank do not have sufficient structural strength to be shipped and installed with the monitor space (annular space) filled with liquid. Currently fiberglass tanks are shipped with a vacuum in the annular space to hold the inner and outer tank shell together preventing separation of either wall from the ribs placed between the walls.
There has now been discovered methods whereby new and used storage tanks with flat ends can be provided with a fiberglass jacket of sufficient strength in the flat end area of the tank to hold a detecting liquid in the space between the storage tank and the newly formed secondary containment area commonly called the annular space. The separating material is capable of adjusting to the shrinkage encountered with fiberglass and resins preventing sealing when the jacket shrinks around the tank's cylinder. Used storage tanks are refurbished to a standard equivalent to that possessed by a new tank and then upgraded to have a secondary containment feature.